Scientific researchers have determined that Harbour Seals can learn to change their voice to make them sound bigger, and that the behaviour is not a result of their anatomy.

Bigger animals usually have deeper (lower pitched) voices than smaller animals, but the Harbour Seal (Phoca vitulina) seems to be different. The Harbour Seal seems to be able to learn to change its voice. 

The Harbour Seal, a marine (saltwater) mammal in the Phocidae family of seals is a pinniped (fin-footed, semi-aquatic mammal such as a seal, sea lion, and walrus) found in the Northern Hemisphere. It is found in the Atlantic Ocean, Pacific Ocean, Baltic Sea, and the North Sea.